Thursday, March 22, 2007

AcoustiSoft - too much fun

Give me a new measuring tool and I am one happy person. I downloaded a demo of AcoustiSoft's measuring software and the room tests that I've been doing with difficulty suddenly become fairly easy.

It runs on Windows only (minus points for that) so I dug out an old computer with Windows 2000 and a sound card. It helps if the sound card is duplex, that is, it can record and play at the same time. This one did - after a driver update.

What drove me to this was the scary prospect of building diffusers for this big room. It's a great looking room, gradually becoming less great looking with all the traps we are installing, and putting diffusers over the brick or wood was not appealing. I knew there was a problem, just from listening to the boxy sound. The click tests that I devised didn't tell me a lot of information. Enter AcoustiSoft!

I set up the speaker in the guitar position. The test starts by chugging out some white sound for 5 seconds, then you click a few buttons to see the results.

The following graph is an impulse followed by high frequency reflections.

For this result, it didn't seem to matter much where I placed the absorbers. This graph always showed a 'bounce' about 2 and a half milliseconds after the initial impulse. I assume this is the floor reflection. Not much to see here.

The next graph is the scary one. See those wild swings in the green line? That's comb filtering, according to their demo site (see D3). I think that's what is causing the boxy sound. I have to find a way to stop that!

Treating the room is helping a lot, but I'm not up to putting diffusers over the walls. A better solution is to isolate the musician in a booth, so after rearranging the absorbers into a 'fort', concentrating on having the panels block the shorter width of the room, I get the following test results:The odd jigger in response at about 8 kHz is a fair trade for the much smoother line below 8000 hertz. Before building the recording 'fort', the swings were as much as 30 db. Here there are less than 10.

The low frequency response, below, is the smoothest of the 4 positions I tried. There is kind of a weird spike at 177 hertz. Hmmm.

I tried the panels with foil side in and foil side out. Once again, I'm surprised to find how much the foil seems to affect low frequency response. It was much smoother with the foil away from me. And that weird spike at 177 hertz was almost inverted and bordered by two spikes.

It's worth mentioning the RT60 tests. This is the 'Reverberation Time' that it takes for a sound to decay 60 decibels. Low reverberation times (well under 500 milliseconds) are good. This is an RT60 graph of the room: